The original urban guerrilla has grown up and is moving to the country.

The Spitalfields Historic Buildings Trust, famed for preventing the wholesale demolition of some of London's loveliest houses by a combination of squatting, sit-ins and the ruthless use of weapons of mass publicity, has decided its job is done. Now the trust is restoring an ancient derelict house in Monmouthshire and is fighting to save houses in Derby and Driffield. The country won't know what's hit it.

The trust has not left Spitalfields entirely, however. It is still monitoring covenants preventing inappropriate development and supplying its unrivalled expertise on restoration, built up during the struggle to save the area.

Back in the Seventies, Spitalfields was rotting. The houses were occupied by sweatshops turning out schmutter for the rag trade. No maintenance had been done for 100 years, and nobody would lend money to buy them.

A number of architectural critics and historians had discovered the area, including Mark Girouard, Colin Amery and Dan Cruickshank. They saw what nobody else did: that Spitalfields was the last surviving enclave of 18th century houses in London and had to be preserved.

Built for rich merchants and Huguenot silk weavers, the houses were of outstanding quality. Heavily carved doorframes opened onto panelled interiors with ornamented staircases and rich plasterwork.

Unfortunately, in Victorian times the area became surrounded by slums and the fashionable residents moved to the West End. Most of the houses were converted into tenements. At Spitalfields' nadir, Jack the Ripper stalked the streets.

'Ironically, the poverty saved the houses,' says Douglas Blain, secretary and one of the founders of the trust. 'Tenants dared not make any alterations to the structure, and the landlords could not afford to.'

The Sixties boom changed all that. The old Huguenot houses were standing in the way of progress.'Spitalfields was seen as an area for the expansion of the City, ripe for demolition and replacement with offices,' he says.

The group formed the trust to try and hold back the City. The aim was to retain clumps of houses, which would stand a better chance of surviving than scattered individual examples.

'We tried to preserve the "bookends" at the ends of terraces so the buildings between would not be vulnerable,' he explains.

Several houses were bought for virtually nothing and gently restored for sale to people who would live in them. Meanwhile, the trust mounted a furious publicity campaign, squatting in two houses in Elder Street and holding that so -Seventies event, a sit-in, at the offices of British Land in Portman Square. The local housing trust, Newlon, was also bullied into either restoring houses or selling them, rather than building new apartment blocks. The trust began to buy threatened houses and restore them. Slowly, the area became trendy.

'We owe an awful lot to the homosex ual community,' Blain says. 'They had money, they wanted somewhere not too conspicuous, but with style. And they didn't have children who would need schooling.'

The turning point was the successful reversal of a huge scheme to build offices all over Spitalfields market and the surrounding streets. The developers, St George, were persuaded to replace a modernistic design for flats in Folgate Street with reproduction 18th century houses, thus preserving the atmosphere of the whole area.

Now the market has been transformed into a Covent Garden-style destination and the area buzzes. Even schools are now available and families are moving back. And houses that were worthless in the Seventies have million-pound price tags.

Some details still need to be attended to, although the trust will not become directly involved except in emergency. Blain himself has just cleaned up one of the less successful restorations, done by a developer at 10 Folgate Street as a quid pro quo for permission to develop the house next door.

Remarkably, almost all of the original house has survived decades of being a shop - even the solemn Doric doorcase, the panelling and the pair of china-cupboards in the front parlour. Unlike some of the more 'fundamentalist' Spitalfields restorations, where the owners use coal fires and candles, the house is fully up-to-date, with a modern kitchen on the ground floor (it would have originally been in the basement), central heating and a lovely bathroom with a roll-top bath.

The three-bedroom house is for sale at £1.3 million through Jackson-Stops & Staff (020 7664 6646).

The most famous street in Spitalfields is Fournier Street - an almost unbroken line of 18th-century houses topped with the famous lofts that were added to provide a well-lit space for the silk-weavers' looms.

Newlon Trust, one of the first landlords to be converted to the path of righteousness, is selling 29 Fournier Street at FPD Savills's next auction on 26 July (call 020 7824 9091 for details). The house is not a perfect specimen - it had new windows installed in Victorian times and is completely unmodernised. Add a recent infestation of squatters and you get the low expected price of £625,000.

Hamptons (020 7626 7726) is selling 18 Fournier Street - beautifully restored with a charming 18th-century-style painting of a lady on the ceiling of the drawing room. It has a south-facing courtyard garden and - very unusually - a garage. The price is £1.25m.

Around the corner in Princelet Street is the house and studio of Neil Simmons, best known for his bust of Margaret Thatcher, of which some studies remain in the house. The house was featured recently in BBC TV's House Detectives , when the team uncovered a fancy redecoration of the main rooms done in the mid-19th century for a Jewish wedding - a rare find. Unfortunately for Simmons, this means the whole ground floor ought to be restored to conservation standards at huge cost. He doesn't have the budget, so is looking for someone with the money and enthusiasm for the project.

Behind the house is a classic example of one of the area's characteristically odd features - a banana-ripening shed. In the 19th century, fruiterers at Spitalfields market took over many of the houses and built glasshouses in the gardens to ripen fruit from the docks. Many survive, and form incredible spaces for entertaining, working out or just working.